1. Field of the Invention
The present invention relates generally to a composite electrical insulator wherein a metal fitting is fixedly secured to one end of a plastic rod which is covered by an insulating sheath.
2. Description of the Related Art
A composite electrical insulator with such a constitution is known, e.g., from U.S. Pat. No. 4,654,478, wherein a fiber-reinforced plastic rod is covered by a sheath comprising a resilient and electrically insulating material, such as silicone rubber, ethylenepropylene rubber and the like, and one end portion of the plastic rod as covered by the sheath is inserted into the bore in a sleeve portion of the metal fitting and the metal fitting is then fixedly secured to the plastic rod. Such a composite insulator makes use of advantages of various materials, e.g., an improved resistance to tensile force and an excellent weight to strength ratio of the fiber reinforced plastic material, and distinguished weatherability and anti-tracking characteristics of silicone rubber, ethylenepropylene rubber or the like.
To manufacture such composite insulators, the sleeve portion of the metal fitting is usually compressed radially inwardly onto the plastic rod so as to firmly clamp the rod. That is to say, by compressing the sleeve portion radially inwardly, the end portion of the plastic rod situated opposite to the metal fitting is uniformly and tightly clamped to integrally connect the metal fitting with the plastic rod and prevent withdrawal of the plastic rod from the fitting even under a large tensile force, while maintaining a water-tight state inner the annular space between the outer surface of the sheath and the inner surface of the sleeve portion of the metal fitting.
Typically, the metal fitting is subjected to a dip-plating so that the outer surface of the metal fitting is more or less uneven. In this instance, an insufficient clamping force tends to form a gap between the metal fitting and the sheath, often making it difficult to preserve the required tightness. A deteriorated tightness results in intrusion of water from outside into the space between the metal fitting and the sheath, and hence in a difficulty to maintain the required electrical insulating property, possibly giving rise to an internal destruction due to flashover.
While a required tightness may be realized by an increased clamping force, the resilient material forming the sheath would then be maintained in an excessively compressed state and thus undergo a gradual deterioration in the restoring characteristic so that it would be impossible to achieve the required sealing function any more. A similar problem may arise also when an increased clamping force causes the sheath to expand radially outwardly to deteriorate the adhesive characteristic of the adhesive material interposed between the sheath and the rod.